Organic electronic elements are elements that carry out an electrical operation using organic substances, and are expected to exhibit features such as energy saving, low price, and flexibility. Thus, more attention is being paid to organic electronic elements as a technology replacing the traditional inorganic semiconductors that are mainly composed of silicon.
Examples of the organic electronic elements include organic EL elements, organic transistors, and organic solar cells.
Among the organic electronic elements, the organic EL elements are attracting attention for their use as, for example, large-sized solid state light sources as a substitute for incandescent lamps and gas-filled lamps. Furthermore, the organic EL elements are also receiving attention as the most promising self-emissive displays substituting for liquid crystal displays (LCD) in the field of flat panel display (FPD), and thus, productization of the organic EL elements is in progress.
The organic EL elements are largely classified into two classes such as low molecular weight type organic EL elements and polymer type organic EL elements, on the basis of the material used and the film forming method. The polymer type organic EL elements are such that since the organic material is composed of a polymeric material, film formation can be conveniently achieved during printing, inkjetting or the like as compared with the low molecular weight type organic EL elements which require a vacuum system for film formation, and therefore, the polymer type organic EL elements are elements indispensable for large-sized organic EL displays of the future.
Vigorous research has been conducted so far on the low molecular weight type organic EL elements and the polymer type organic EL elements, but there still are problems of low light emission efficiency and a short element lifetime. As a means to address these problems, the low molecular weight type organic EL elements are fabricated to have a multilayer structure.
FIG. 1 shows an example of a multilayered organic EL element. In FIG. 1, a layer which is in charge of light emission is indicated as a light emitting layer 1, and in case that the organic EL element has other layers, a layer that is in contact with an anode 2 is indicated as a hole injection layer 3, while a layer that is in contact with a cathode 4 is indicated as an electron injection layer 5. Furthermore, if another layer is present between the light emitting layer 1 and the hole injection layer 3, the layer is indicated as a hole transport layer 6. If another layer is present between the light emitting layer 1 and the electron injection layer 5, the layer is indicated as an electron transport layer 7. In FIG. 1, reference numeral 8 indicates a substrate.
Since film formation during the production of low molecular weight type organic EL elements is dominantly carried out by a vapor deposition method, a multilayer structure can be easily achieved by performing vapor deposition while sequentially changing the compounds used. On the other hand, in the case of the polymer type organic EL elements, since film formation is carried out using a wet process such as printing or inkjetting, there occurs a problem that when an upper layer is applied, the lower layer undergoes dissolution.
Therefore, providing a multilayer structure in the polymer type organic EL elements is more difficult than in the case of the low molecular weight type organic EL elements, and the effects of enhancing the light emission efficiency and improving the lifetime could not be obtained.
In order to cope with this problem, several methods have been hitherto suggested. One of them is a method of using the difference in solubility. For example, an element having a two-layer structure of a hole injection layer formed from water-soluble polythiophene:polystyrene sulfonate (PEDOT:PSS), and a light emitting layer formed by using an aromatic organic solvent such as toluene. In this case, since the PEDOT:PSS layer does not dissolve in an aromatic solvent such as toluene, it is possible to produce a two-layer structure.
However, when water-soluble PEDOT:PSS is used, the moisture remaining in the thin film needs to be removed, and this removal of moisture is difficult and causes deterioration of the properties of the organic electronic element. Further, for the removal of moisture, it is necessary to dry the element at high temperature for a long time period, so that production of an organic electronic element on a resin substrate is difficult, or significant restrictions are imposed on the process, such as reduced pressure conditions.
Furthermore, there has been disclosed, as an example using an organic solvent, a method of selecting a solvent that does not affect a lower layer that has been formed earlier (see Patent Document 1).
However, in such a method, the solvent that can be used is limited to a solvent that does not dissolve the lower layer, and therefore, there is a problem that only a narrow choice of materials is available. Further, a certain degree of erosion occurs in the lower layer at the time of the formation of an upper layer.
Furthermore, Non-Patent Document 1 suggests an element having a three-layer structure which uses compounds having largely different solubility.
Patent Document 2 also discloses an element having, on a PEDOT:PSS layer, a three-layer structure into which a layer called interlayer has been introduced.
In order to overcome such problems, Non-Patent Documents 2 to 4 and Patent Document 3 disclose methods of making a thin film insoluble to a solvent by utilizing a polymerization reaction of a siloxane compound, an oxetane group, a vinyl group or the like and thereby changing the solubility of the compound.
Methods for providing a multilayer structure as such are important. However, there is the problem described above that is attributable to the moisture remaining in the thin film when water-soluble PEDOT:PSS is used, or there are problems that there are restrictions on the material that can be used, in order to utilize the difference in solubility, that siloxane compounds are unstable to moisture in air, and that the properties of the element are not satisfactory.
Further, in the case of utilizing a polymerization reaction, it is necessary to add an appropriate polymerization initiator that generates an acid, a base, a radical or the like, and to thereby initiate the polymerization reaction through stimulation such as light or heat.
As a cause for initiating the polymerization reaction, heating or a combined use of light irradiation and heating is generally used, and in order to bring the polymerization reaction to a sufficient extent, it is needed to heat the reaction system at a temperature of 120° C. or higher (Non-Patent Document 4 and Non-Patent Document 5).
Here, there is a need to apply a substrate made of an inexpensive and flexible resin in the production of a flexible organic EL element in view of reducing the production cost for organic EL elements. However, since such a substrate undergoes softening, decomposition or degeneration due to high temperature, there is a problem that the method of bringing the polymerization reaction cannot be utilized.
In addition, a method of using a crosslinking reaction has been suggested as another method of producing a multilayer structure. Patent Document 4 discloses a method of crosslinking triphenylamine-containing ether polyether ketone by ultraviolet irradiation, and thereby making the compound insoluble. In order to make the compound sufficiently insoluble through this method, there is a problem that ultraviolet irradiation for a long time period is required, and decomposition of triphenylamine or the like occurs.
Furthermore, Patent Document 5, Patent Document 6, Non-Patent Document 6, and Non-Patent Document 7 disclose the production of a multilayer structure through crosslinking of an oxetane group. In these methods, photoinitiators are used, and therefore, there is a concern for deterioration due to light. Furthermore, there is a problem that sufficient insolubilization at low temperature does not proceed, and, and the application of resin substrates which require low temperature curing is restricted, or there is a problem that at the time of forming an upper layer, the upper layer and the lower layer are intermixed, causing deterioration of the organic EL characteristics. Moreover, the photoinitiators used in these methods are general iodonium salts or sulfonium salts, and there is a concern for the influence of these compounds on the EL characteristics.
On the other hand, an investigation is being conducted on the use of an iodonium salt or sulfonium salt having the same structure as that of the photoinitiators in the hole transport layer or the light emitting layer, for the purpose of lowering the driving voltage, which is a problem for organic EL elements.
Patent Document 7 discloses an ionic compound, but this has the same structure as that of the photoinitiator described above, and thus there is a concern for the influence of the compound on the properties of the organic EL elements. Furthermore, there are no descriptions on crosslinking or lamination.
Patent Document 8 discloses a polymer illuminant composition containing a polymer illuminant and an ion pair. There is a description that a light emitting element having a much longer lifetime may be obtained by incorporating an ion pair having a specific structure according to the disclosure, but there are no descriptions on the injection and transport of charges. There are also no descriptions on crosslinking or lamination.